Electromagnetically driven actuators can be operated with low tolerance in the so-called full-stroke operating mode. This means that an armature of the actuator is moved to and fro between an initial position and an end position. In this case, the initial position and the end position are typically each defined by a mechanical stop of the armature on a housing of the actuator. Using the example of an injection valve for fuel injection, this operating mode means that a valve needle of the injection valve is moved in each case up to a maximum deflection. A variation of the injected fuel quantity then takes place by appropriately matching the duration of the injection process.
In order to reduce emissions of pollutants and/or the fuel consumption of motor vehicles, it is necessary, however, in modern injection systems, to manage the operation of injection valves as accurately as possible even in the case of small injection quantities. This means that the so-called ballistic operation of an injection valve is also managed. Ballistic operation of an injection valve is understood in this context to mean a partial deflection of the armature or the valve needle in a trajectory which is predetermined by electrical and/or design parameters and is free once the electromagnetic force introduction onto the armature is complete, i.e. parabolic trajectory, without reaching the full stop.
In contrast to the full-stroke operation, the ballistic operation of an injection valve is subject to markedly more tolerances since, in this case, both electrical and mechanical tolerances influence the opening profile to a considerably greater extent than is the case in the full-stroke operation.
For the ballistic operating mode of an injection valve, generally of an electromagnetically driven armature of an actuator having a coil, in this case the following tolerances can occur individually or in combination:
a) Opening tolerance: the time at which the armature moves away from its initial position once a defined electrical drive pulse has been applied to the coil is dependent on the electrical, magnetic and/or mechanical properties of the individual injection valve and/or on the operating state thereof (for example temperature).
b) Closing tolerance: the time at which the armature returns to its initial position after a partial deflection is dependent on the electrical, magnetic and/or mechanical properties of the individual injection valve and/or on the operating state thereof.
c) Stroke tolerance: in the case of a partial deflection of the armature, the maximum stroke reached is likewise dependent on the electrical, magnetic and/or mechanical properties of the individual injection valve and/or on the operating state thereof. The stroke tolerance results in an individual change in the parabolic trajectory of the armature with the possibility of leveling-off or peaking of the corresponding deflection curve.
DE 10 2006 035 225 A1 discloses an electromagnetic actuating apparatus which has a coil. By virtue of evaluation of induced voltage signals which are caused by external mechanical influences, the actual movement of the actuating apparatus can be analyzed.
DE 198 34 405 A1 discloses a method for estimating a needle stroke of a solenoid valve. During the movement of the valve needle relative to a coil of the solenoid valve, the voltages induced in the coil are detected and set in relation to the stroke of the valve needle by means of a computation model. In order to ascertain the contact time, the time derivative dU/dt of the coil voltage can be used since this signal has very sudden changes at the reversal point of the needle or armature movement.
DE 38 43 138 A1 discloses a method for controlling and detecting the movement of an armature of an electromagnetic switching element. On disconnection of the switching element, a magnetic field is induced in the field winding of said switching element, which magnetic field is changed by the armature movement. The changes over time in the voltage present at the field winding which occur on the basis of this can be used to detect the end of the armature movement.